Circuit architecture for electro-optic modulation based on free carrier dispersion effect and the waveguide capacitor structures for such modulator circuitry using CMOS or Bi-CMOS process

ABSTRACT

New circuit architecture for electro-optic modulator based on free-carrier dispersion effect is invented, in which the waveguide capacitor of the modulator is embed in the circuits and physically layout together with transistors, the switching of the modulator occurs in transistors, and as the result, the electro-optical modulation occurs in the waveguide capacitor. The invented modulator is not one physical device, it is actually a circuit. Several circuit design techniques are imported, leading to several new modulator circuits that have very high operation speed and very small power consumption. Several new waveguide capacitor structures are also invented that allow high efficient modulator circuits to be built.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/756,490, filed May 31, 2007, which claims the benefit of U.S.Provisional Patent Application No. 60/810,684, filed Jun. 1, 2006, theentire contents of which are hereby incorporated by reference in theirentirety.

BACKGROUND OF THE INVENTION

The external high-speed electro-optic modulator is a necessary devicefor varieties of optical communication systems, where the directmodulation of the laser is not applicable due to either operation speedlimitation or the frequency chirp or poor signal integrality causingunbearable impairment during the transmission.

Being able to fabricate the electro-optic modulator on silicon usingstandard VLSI process possesses the advantages of low-cost and easyintegration with electronics. Recently, extensive efforts of developingelectro-optic modulator on silicon have been taken, and lots ofbreakthrough development. The published works include forward biased PINdiode structure^([1][2]) and metal-on-semiconductor (MOS) capacitancestructure^([3]), which was able to demonstrate 1 Gbps operation speed.The inventor of this patent has made the most critical originalcontribution to this field, and successfully demonstrated the first 10Gbps silicon EO modulator fabricated standard CMOS process. The attachedpatent documents^([4-6]) presentpart of the silicon modulatortechnologies that the inventor have developed in the past.

-   1. P. D. Hewitt and G. T. Reed, “Improved modulation performance of    a silicon p-i-n device by trench isolation,” J. of Lightwave    Technology, Vol. 19, No. 3, pp 387-390, March 2001.-   2. Giuseppe Coppola, Andrea Irace, Mario Iodice, Antonello Cutolo,    “Simulation and analysis of a high-efficiency silicon optoelectronic    modulator based on a Bragg mirror,” Opt. Eng. 40(6), 1076-1082 (June    2001).-   3. “A high speed silicon optical modulator based on    metal-oxide-semiconductor capacitor”. Nature, vol. 427, 2004, page    615-618.-   4. Integrated optical and electronic devices (CMOS Optics). Luxtera    Inc., application Ser. No. 10/606,297.-   5. Active waveguides for optoelectronic devices. Luxtera Inc.,    application no. 10/650,234.-   6. High-speed electro-optic modulator on silicon SOI using    periodical distributed lateral abrupt PN diode structure. Luxtera    Inc., provisional application No. 60/495,402.

BRIEF SUMMARY OF THE INVENTION

All the previous designs and inventions have regarded the EO modulatoras a device, in which the physical device of the modulator is separatedfrom its driving electronics. This conventional technique does not takethe advantage of the monolithic integration between transistors andoptical waveguides. The performance is fundamentally limited due to theseparation of the driving electronics and the optical waveguide itself.

In this invention, I discard this conventional design concept. The EOmodulators invented in this application are actually circuits: theelectrical-optical interaction occurs in the waveguide-capacitor, whichis seamlessly, in one body or distributive manner, part of amodulator-circuit. In such way, many existing circuit design techniquescan be used in the EO modulator design. The invention will show that themodulator based on free-carrier dispersion design is noting but aswitching circuit design. This new design approach and the modulatorcircuits can improve the performance of the modulator to its physicallimit, in both operation speed and power consumption.

The invention comprises the design approach, fundamental structure, anda variety of high performance EO modulator-circuits based onfree-carrier dispersion effect, and several new waveguide capacitorstructures that can be used the modulator circuits.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an embodiment of a modulator circuit.

FIG. 2 shows an embodiment of a modulator circuit in greater detail.

FIG. 3 shows an embodiment of a switched-oscillator modulator circuit.

FIG. 4 shows an embodiment of a switched-oscillator modulator circuit ingreater detail.

FIG. 5 shows a simulation result of the modulator of FIG. 4.

FIG. 6 shows an I-type waveguide capacitor embodiment and a Y-typewaveguide capacitor embodiment.

FIG. 7 shows capsulated waveguide capacitor embodiments.

FIG. 8 shows an embodiment of a bipolar transistor junction waveguidecapacitor.

FIG. 9 shows a slow-wave transmission line electrode in an embodiment ofa slow-wave modulator circuit.

FIG. 10 shows an embodiment of a slow-wave optical waveguide.

FIG. 11 shows an embodiment of an optical waveguide with grating.

DETAIL DESCRIPTION OF THE INVENTION

FIG. 1 illustrates the invented modulator circuit in concept, in thefigure:

<0>: the input light to the electro-optical modulation circuit;

<1>: the output light from the electro-optical modulation circuit, whichhas been modulated by the driving electrical signal;

<3>: the optical waveguide body, in which the lightwave is guided(confined);

<4>: the input (driving) electrical signal (data);

<5>: the code conversion circuit;

<6>: controlled current source;

<7>: the waveguide capacitor

symbol representative in the electro-optic modulation circuit;

<9>: the waveguide capacitor, in which is the common physical body ofthe optical waveguide and the modulator-capacitor;

<10>: the self-discharge path of the waveguide capacitor;

The capacitor inside the waveguide cross section represents all types ofpossible structures (PIN, reverse PN, and MOS, etc.), plus the newmodulator-capacitor structures that will be described in thisapplication. Those new modulator-capacitor structures are also part ofthis invention.

In circuits, the way to charge a capacitor is to pump current throughit. The charge accumulated inside capacitor can be solved by thefollowing differential equation is:

$\begin{matrix}{\frac{Q}{t} = {{i_{d}(t)} - \frac{Q}{{C(Q)} \cdot R}}} & (1)\end{matrix}$

where i_(d)(t) is the charging current from the voltage-controlledcurrent source in the modulator circuit. The R represents theself-discharge path <10>. It could be the parasitic parallel resistanceof the capacitor. It could also be the self-recombination of the holeand electrons inside the PIN junction, if so, the C(Q)·R should bereplaced by the time constant determined by the minority carrierlifetime.

Since the carrier density inside the waveguide volume is directlyproportional to the charge Q, which determines the refractive index ofthe semiconductor material, silicon or III-V, by the free carrierdispersion effect, the charge Q in equation (1) actually represents thephase (intensity) of the lightwave after the modulator waveguide(interferometer).

We find that, as presented by FIG. 1, the modulator design problem isactually a circuit design problem. What we need to do is to design thecircuit to produce the i_(d)(t) such that the solution of the equation(1) is the data waveform required. To avoid propagation delay, thevoltage-controlled current source circuit <6> must have ignorabledistance from the waveguide capacitor <9>. In addition, as one will findlater, the voltage signal v_(d) (t) usually needs to be pulse signalthat has much wider bandwidth than the input data. So, the codeconversion circuit <5> must stay together with the rest of modulatorcircuits as well. This circuit-based modulator is only possible when wecan realize the monolithic integration of electronic circuits andoptical waveguide.

Using CMOS or SiGe BiCMOS process, we can monolithic integrateelectronic circuits and photonic waveguide circuits together.

Invention Embodiment 1 Edge-Triggered One-Shot Circuit Driving ChargePump Modulator Circuit

FIG. 2 gives an edge-triggered one-shot circuit driving charge pumpmodulation circuit, in the figure:

<11>: the input electrical data, which is a non-return-zero (NRZ) code;

<12>: the output of the code conversion, corresponding to the risingedge of the input data <11>;

<13>: the output of the code conversion, corresponding to the fallingedge of the input data <11>;

<14>: rising edge triggered one-shot circuit, which generates <12>;

15>: falling edge triggered one-shot circuit, which generates <13>;

<16>: charge pump circuit to charge the waveguide capacitor of themodulation circuit;

<17>: charge pump circuit to discharge the waveguide capacitor of themodulation circuit;

<18>: the waveform of the charge accumulated in the waveguide capacitorQ(t);

If the waveguide capacitor uses the majority carrier of thesemiconductor, the self-discharge path can be ignored; the equation (1)can be simplified to:

$\begin{matrix}{\frac{Q}{t} = {i_{d}(t)}} & (2)\end{matrix}$

The required Q(t) is a NRZ data waveform. The code conversion circuitscan be an edge triggered one-shot circuit <14><15>, and thevoltage-controlled current source can be a simple charge pump circuit<16><17> as shown in FOG. 2. The input NRZ data <11> drives two edgetriggered one-shot circuits, one responds to the rising edge of thedata, and the other responds to the falling edge of the data. The pulsewidth of one-shot

output can be adjusted by the delay time τ. After the code conversion(two parallel edge triggered one-shot circuits), two parallel v_(d) (t)signals are generated, each controls half of the charge pump. Theresulted Q(t) waveform becomes the NRZ data as needed, with the rise andfall time equal to the pulse width of the v_(d) (t).

Invention Embodiment 2 Low Power-Consuming Electro-Optical ModulatorCircuit Using Resonator Switching

Since changing the refraction index of silicon is realized bymanipulating the carrier density (either majority carrier density orminority carrier density), driving the modulator is actually charging ordischarging the semiconductor junction capacitor or other capacitancestructure in semiconductor (for example, MOS capacitor). In travelingwave devices, the electrical energy will propagate along the device andeventually dissipated at the termination end. The device invented herewill allow us to re-use that energy for the following bits. Anotherdescription for this is, assuming always drive the modulator byreturn-to-zero (RZ) data, when the charging and discharging finished forone bit (logic 1 at that current bit), the energy released from themodulator-capacitor will be stored in somewhere else. If the followingbit is also logic 1, the stored energy will be injected back to themodulator-capacitor and then released (charging and discharging) again.If the following bit is logic 0, then do nothing (keep holding theenergy in that

omewhere else?.

The way of realize such procedure is to construct a resonator withswitched capacitor, as shown in FIG. 3, in the figure:

<30>: the power to maintain the oscillation;

<31>: the inductor L1;

<32>: the waveguide capacitor formed by diode junction (Dmod);

<33>: the image capacitor (Cdummy) of the waveguide capacitor (Dmod),Cdummy needs to have the exact same value as <32>;

<34>: the switch for the resonator circuit of the modulation (X1);

<35>: the switch for the image resonator circuit (X2);

The way it works is, assuming at the first oscillation cycle, the Dmodis charged to the top voltage level and then discharged completely, atthat moment, the current flowing through the inductor L1 is at themaximum (electrical field energy in Dmod is released and stored in L1 asmagnetic field energy). In the following next bit period, if the bit islogic 0, X1 will be switched off and X2 will be switched on, therecharging will be performed from L1 to Cdummy. The cycle will becontinued until another logic 1 comes in, then X1 will be switched onand X2 will be switched off. The oscillation cycle goes back between L1and Dmod. Either X1 or X2 will be switched on at any given time but notboth. The Vsource is just a power source to maintain the oscillation toovercome the parasitic ohm loss.

FIG. 4 illustrates a practical implementation of such device, in thefigure:

<41>: the waveguide capacitor 1 (Cnmod), which is one of the arms of theMach-Zehnder Interferometer;

<42>: the waveguide capacitor 2 (Csmod), which is another arm of theMach-Zehnder Interferometer;

<43>: the same as <34>;

<44>: the same as <35>;

<45>: the parasitic resistance in the resonator circuits;

<46>: the input electrical signal controlling <43>;

<47>: the input electrical signal controlling <44>;

<48>: the power source supplying power to the resonator circuits;

<49>: the terminal 1 of the waveguide capacitor (Cnmod);

<50>: the terminal 2 of the waveguide capacitor (Cnmod);

In practice, the Cdummy <33> in FIG. 3 can be replaced by anotherwaveguide arm of the Mach-Zehnder interferometer (MZI) of the modulator,which is the case in FIG. 4: the Cnmod <41> represent themodulator-capacitor on north arm of the MZ interferometer, and the Csmod<42> represent the modulator-capacitor on the south arm of the MZinterferometer. Due to the fact that the data applied on the two arms ofthe MZI are always complementary, whenever Cnmod is charged, Csmod shallnot be charged. The oscillator is using the source coupled topology. Thecurrent source Ml is pumping the LC resonator.

FIG. 5 is the simulation result of such switched oscillated modulator asshown in FIG. 4, in the figure:

<51>: the electrical potential level difference between <49> and <50> inFIG. 4, which is the voltage across the waveguide capacitor <41> in FIG.4;

<52>: the electrical potential level of <49> in FIG. 4;

<53>: the electrical potential level of <50> in FIG. 4;

In this simulation, the electro-optic modulation circuit in FIG. 4 isdriven by RZ data. The resulted voltage <51> across the waveguidecapacitor <41> represents the charge stored in the waveguide capacitorat any transient time. Therefore, <51> also represents the optical phasemodulation in the free-carrier dispersion based electro-optic modulator.

Invention Embodiment 3 Waveguide Capacitor for Electro-Optic ModulatorCircuits

Anther part of this invention is three new modulator waveguide capacitorstructures. They are: (1) the I-type or Y-type of waveguide with gatecapacitor (FIG. 6); (2) capsulated waveguide capacitor (FIG. 7); (3)bipolar transistor junctions (FIG. 8).

The I-type or Y-type of waveguide with MOS capacitor is shown in FIG. 6,in the figure:

<61>: silicon area I;

<62>: metal contact;

<63>: gate oxide or other gate insulator in MOS;

<64>: the optical mode illustration in the waveguide of the modulator;

<65>: silicon area II;

<66>: passivation oxide, also the top cladding of the optical waveguide;

<67>: field oxide of the MOS transistor;

For the I-type waveguide capacitor, as shown in the left of FIG. 6, thesurface is CMP to flat after the field oxide deposition or thermalgrowth. The silicon-II <65> is the original silicon layer of the SOIwafer, and the silicon-I can be the poly silicon deposited, or singlecrystal silicon grown, on top of finished surface after gate oxide andfield oxide. For the Y-type waveguide capacitor, as shown in the rightof FIG. 6, the field oxide is the result of the selective thermaloxidation, and no CMP process after it. The poly silicon <71> is thendeposited on top.

In both I-type and Y-type, there is a thin gate oxide <63> thermal grownbefore poly silicon deposition or silicon-I growth. This thin gate oxideguarantees the high capacitance between <61> and <65>, and between <71>and <65>.

The I-type/Y-type waveguide is generally a waveguide structures thathave the high index ridge existing between two high index slabs. The tophigh index slab can be flat, or not flat according to the process.No-flat top slab is usually an advantage to form a more lateral confinedguiding mode. The lateral confinement to the lightwave is provided bythe effective index difference just like in the regular ridge waveguide.The mode will be mainly confined at the center part defined by theridge. Therefore, the contacts to the both slabs with enough distancefrom the center will have no effect to the mode.

The capsulated waveguide capacitor is shown in FIG. 7, in the figure:

<71>: poly silicon layer;

<72>: single crystal silicon layer, which is the silicon layer of theSOI wafer;

<73>: capsulated ridge waveguide capacitor;

<74>: capsulated Y-type waveguide capacitor;

In <73>, after the gate oxide growth or deposition, the silicon ridge iscapsulated by the poly silicon film <71>. The shape of the ridge isreserved after capsulation. Therefore, the lateral confinement isreserved as well. The contact away enough from the ridge has no effectto the guiding mode. And the thin gate oxide makes sure the sufficientcapacitance between the poly silicon and the crystal silicon layerunderneath.

In <74>, it is a Y-type waveguide with a slot in the middle of thesilicon ridge. After the gate oxide growth or deposition and the fieldoxide deposition (outside the slot), the slot is filled by the polysilicon film <71>, which is conductive. The capacitor exists between thepoly film and the crystal silicon underneath the gate oxide. The pros ofthis structure are that the capacitor is more overlapped with the centerof the optical guiding mode, and the field oxide make the poly siliconand crystal silicon further apart (therefore, less capacitance) at thelocation where no significant optical field due to the lateralconfinement of the guiding mode.

FIG. 8 is the illustration of the third modulator capacitor: the bipolartransistor junction, in the figure:

<81>: the base of the lateral PNP bipolar transistor;

<82>: the emitter contact of the lateral PNP transistor, also the basecontact of the lateral NPN transistor;

<83>: the collector contact of the lateral PNP transistor;

<84>: the NPN section of the waveguide capacitor;

<85>: the PNP section of the waveguide capacitor;

<86>: the field oxide of the SOI-CMOS;

<87>: the ridge of the SOI ridge waveguide;

In this bipolar transistor junction structure, the complementary lateralbipolar transistor pair is made out of the ridge waveguide by implants.Using the field oxide as implant mask, the base implant can beself-aligned. A base width as narrow as top ridge width can be achieved.In this bipolar transistor modulator structure, the base width needs tobe as narrow as possible to improve the operation speed. The waveguidewill be designed and fabricated in this way to make the base widthnarrow. One effective way is to use the selective thermal oxidation toform the field oxide and the waveguide ridge. The top of the base(waveguide ridge), can have more density of doping to reduce theresistance, however, the trade-off needs to be played between resistanceand the waveguide loss.

As we can see from the top view in FIG. 8, the both side of the ridgeare implanted in N-type. Heavier doping needed in the area further awayfrom the ridge to improve the quality of the contact and reduce theseries resistance of the emitter and the collector (for the NPNtransistor). For the NPN transistor, the electron flows from the left toright (from emitter to the collector). The base of the NPN is at thecenter of the ridge, and its contact doping and contact itself of theNPN is on the right side of the ridge (as an example), which directlyconnects to the waveguide ridge. On the left side, another P-type dopingarea exists, it is there to turn the parasitic base-emitter current ofthe NPN transistor into a collector current of a PNP transistor. Pleasenotice that the P-type doping on the left (the collector of the PNP)does not contact the ridge base, and there is a narrow N-type dopingbetween them, which functions as the base of the PNP.

By the complementary bipolar transistor pair, the slow parasiticemitter-base current of the NPN can be turned to a fast collectorcurrent of PNP.

By the self-aligned base doping, this bipolar transistor modulator canoperate very fast. As a bipolar transistor by itself, it brings the gaininto the modulator junction. The voltage-controlled current source inFIG. 1 only needs to drive the base capacitance, while the overallcharging is then amplified by the bipolar transistor itself. The chargecurrent is β times larger than the base current coming from the drivingcurrent source, where β is the current gain of the bipolar transistor.

Invention Embodiment 4 Traveling Wave Modulator Circuit Utilizing SlowWave Structure

The invention embodiment here is to construct slow traveling wavestructure for both electrical signal and lightwave in the modulatorcircuit. The electrical slow traveling wave structure is aquasi-transmission line, as shown in FIG. 9. The black solid pattern isthe electrode, the p-doped region and n-doped region of thesemiconductor, along with the optical waveguide underneath the electrodeare shown in the figure by different filling pattern. The dopedsemiconductor (silicon in this embodiment) will be electrical connectedthrough the contacts. The optical waveguide position is indicated. Butthe exact geometry of the waveguide will be described later. In FIG. 9:

<91>: the slow-wave transmission line constructed by inductor-capacitorchain driving the 1^(st) waveguide arm <97> of the MZI, in the saidinductor-capacitor chain, the capacitor comprises both the capacitanceof the metal electrode and the capacitance of the waveguide capacitor(PN junction in this embodiment) of the modulator;

<92>: the slow-wave transmission line constructed by inductor-capacitorchain driving the 2^(nd) waveguide arm <94> of the MZI, in the saidinductor-capacitor chain, the capacitor comprises both the capacitanceof the metal electrode and the capacitance of the waveguide capacitor(PN junction in this embodiment) of the modulator;

<93>: the common ground line of <91> and <92>;

<94>: the 2^(nd) waveguide arm of the Mach-Zehnder Interferometer;

<95>: the inductor of one segment of the inductor-capacitor chain;

<96>: the capacitor (electrode part) of the one segment of theinductor-capacitor chain;

<97>: the 1^(st) waveguide arm of the Mach-Zehnder Interferomeer;

In FIG. 9, the section length L_(s) needs to be small enough compared tothe electrical wavelength of the highest frequency in the driving signal

bandwidth.

To make this slow electrical traveling wave structure can be appliedinto the modulator, the optical waveguide must be a slow opticaltraveling wave structure, in which longitudinal traveling speed of thelightwave matches with the traveling speed of the electrical signal inthe slow-wave transmission line. There are varieties of the approachesto realize such slow-wave optical structure.

FIG. 10 shows one example of the slow-wave optical waveguide: aperiodically bended optical waveguide, in the figure:

<101>: periodically bended optical waveguide as slow-wave waveguidecapacitor;

<102>: the trace of optical waveguide, illustrated by the waveguideridge in the slow-wave optical structure embodiment;

<103>: the P-type doping area of the waveguide capacitor (PN junction inthis slow-wave waveguide capacitor embodiment);

<104>: the N-type doping area of the waveguide capacitor (PN junction inthis slow-wave waveguide capacitor embodiment);

FIG. 11 shows another example: an optical waveguide with grating, in thefigure:

<111>: the waveguide grating;

<112>: the region in the photon energy band structure of the waveguidegrating (1-dimensional photonic crystal) where the velocity of thephoton is approaching to zero; The curve section outlined by the red box<112> has very small

${V_{og} = \frac{\omega}{\beta}},$

which is the group velocity of the guiding mode along the waveguidegrating.

One of the most important advantages of having slow wave traveling wavestructure is to reduce the power consumption of the modulator circuit.Due to the charge accumulated in the waveguide capacitor is proportionalto the voltage between its two terminals, low power consumption willrequire high impedance of the electrical traveling wave structure, whichin turns, require high distributive inductance. However, increasing thedistributive inductance in conventional transmission line willinevitably slow down the electrical signal propagation. By slowing downthe lightwave propagation as well, the group velocity of the light canbe re-aligned with the propagation velocity of the electrical signalalong the electrode after its inductance is intentionally increased,therefore maintain the bandwidth of the modulator circuit.

1. The new waveguide capacitor structure that can enhance theperformance of the electro-optical modulator circuit using free-carrierdispersion effect (FIG. 6, FIG. 7), the said waveguide capacitorstructures comprising two layers of silicon with a gate oxide or othergate dielectric in between, and the lateral confinement of the lightwaveis provided by various structures including: Capsulated ridgestructures, wherein the seed ridge structure is formed by the partiallyor fully etching of the initial silicon film of the SOI wafer, then theridge shape is preserved after the second silicon layer deposition;Capsulated Y-shape waveguide, wherein one or multiple slots are etchedin the seed ridge structure formed from initial silicon layer, and then,after the gate oxide or other insulator dielectric is formed on top ofthe silicon layer, the second silicon layer will deposited into thoseslots, the said second silicon layer extends out to reach the contract,and the said extending can go over the field oxide or below.
 2. The newwaveguide capacitor structure (FIG. 8) comprising A plurality ofcomplementary lateral bipolar transistor pairs along the opticalwaveguide; The base regions of the lateral bipolar transistors, both NPNand PNP, are aligned with the core of the optical waveguide (ridgeregion if it is ridge waveguide, for instance). The base and the basecontact of the NPN transistor is also the emitter and emitter contact ofthe PNP transistor in the same bipolar pair, in such way that the slowbase current of the NPN is turned to become the collector current of thePNP. The base of the NPN is the active region of the waveguide. Thecharging (and discharging) to the said base of the NPN is initiated bythe modulation circuit and is amplified by the gain of the said NPNtransistor itself. The PNP is complementary to NPN in aboveconfiguration. The configuration of NPN and PNP can be reversed also,meaning that the NPN can be the complementary to the PNP.